In recent years, automation in various types of tests such as clinical laboratory tests and reduction in the assay time thereof have been tried. As a method of these tests, an assay utilizing an immune reaction is widely used for measurement of a substance in a biological sample. Examples of the immunoassay include many methods such as RIA, EIA, immunonephelometry, latex agglutination, colloidal gold agglutination, and immunochromatography. Among such methods, the latex agglutination and the colloidal gold agglutination are capable of measurement in a homogeneous system in which the separation or washing operation of a reaction mixture is not required, and therefore suitable for automation of determination and short-time assay. In particular, colloidal gold particles have a size of 5 nm to 100 nm, which is smaller than the size of latex particles, so that colloidal gold particles can be used in an assay of a tracer substance (Japanese Laid-Open Patent Publication Nos. 2005-283250 and 2004-325192).
The main reactive component in these assays is a substance that specifically reacts with the analyte bound to the latex particles or the colloidal gold particles. This substance that specifically reacts with the analyte varies depending on the analyte, and thus it is necessary to prepare specific microparticles having such a substance bound thereto in accordance with the analyte.
However, the operation of preparing such microparticles is complicated. Furthermore, some types of substance to be bound to the microparticles may not easily bind to a carrier, may nonspecifically undergo autoagglutination even when being bound to the carrier and thus cannot be used in an assay system, or may change in the specificity or the avidity to the analyte due to binding and thus cannot be used.